area is designated for the vehicles to turn to different directions to reach
their desired destinations. Its main function is to guide vehicles to their
respective directions. Traffic intersections are complex locations on any
highway. This is because vehicles moving in different direction wan to occupy
same space at the same time.
In addition, the pedestrians also seek same space
for crossing. Drivers have to make split second decision at an intersection by
considering his route, intersection geometry, speed and direction of other
vehicles etc. A small error in judgment can cause severe accidents. It also
causes delay and it depends on type, geometry, and type of control.
traffic flow depends on the performance of the intersections. It also affects
the capacity of the road. Therefore, both from the accident perspective and the
capacity perspective, the study of intersections very important for the traffic
engineers especially in the case of
types of intersection. Consider a typical four-legged intersection as shown in figure.
The number of conflicts for competing through movements are 4, while
competing right turn and through
movements are 8. The conflicts between right turn traffics are 4, and between
left turn and merging traffic is 4. The conflicts created by pedestrians will
be 8 taking into account all the four approaches. Diverging traffic also
produces about 4 conflicts. Therefore, a typical four legged intersection has
about 32 different types of conflicts.
conflicts at the intersection for the safe and efficient movement of both
vehicular traffic and pedestrians. Two methods of intersection controls are
there: time sharing and space sharing. The type of intersection control that
has to be adopted depends on the traffic volume, road geometry, cost involved,
importance of the road etc.
Levels of intersection control
levels. They can be either passive control, semi control, or active control. In
passive control, there is no explicit control on the driver . In semi control,
some amount of control on the driver is there from the traffic agency. Active
control means the movement of the traffic is fully controlled by the traffic
agency and the drivers cannot simply maneuver the intersection according to his
required. Here the road users are required to obey the basic rules of the road.
Passive control like traffic signs, road markings etc. are used to complement
the intersection control. Some of the intersection control that are classified
under passive control are as follows:
low, then by applying the basic rules of the road like driver on the left side
of the road must yield and that through movements will have priority than
turning movements. The driver is expected to obey these basic rules of the
etc. it is able to provide some level of control at an intersection. Give way
control, two-way stop control, and all-way stop control are some examples. The
GIVE WAY control requires the driver in the minor road to slow down to a
minimum speed and allow the vehicle on the major road to proceed. Two way stop
control requires the vehicle drivers on the minor streets should see that the
conflicts are avoided.
difficult to differentiate between the major and minor roads in an
intersection. In such a case, STOP sign is placed on all the approaches to the
intersection and the driver on all the approaches are required to stop the
vehicle. The vehicle at the right side will get priority over the left
approach. Thetraffic control at ’at-grade’ intersection may be uncontrolled in
cases of low traffic. Here the road users are required to obey the basic rules
of the road. Passive control like traffic signs, road markings etc. are used to
complement the intersection control.
signs, road markings also complement the traffic control at intersections. Some
of the examples include stop line marking, yield lines, arrow marking etc.
guided to avoid conflicts. Channelization and traffic rotaries are two examples
raising a portion of the road in the middle usually called as islands
distinguished by road markings. The conflicts in traffic movements are reduced
to a great extent in such a case. In channelized intersections, as the name
suggests, the traffic is directed to flow through different channels and this
physical separation is made possible with the help of some barriers in the road
like traffic islands, road markings etc.
made to flow along one direction around a traffic island. The essential
principle of this control is to convert all the severe conflicts like through
and right turn conflicts into milder conflicts like merging, weaving and
diverging. It is a form of ‘at-grade’ intersection laid out for the movement of
traffic such that no through conflicts are there. Free-left turn is permitted
where as through traffic and right-turn traffic is forced to move around the central
island in a clock-wise direction in an orderly manner. Merging, weaving and
diverging operations reduces the conflicting movements at the rotary.
follow the path suggested by the traffic control agencies. He cannot maneuver
according to his wish. Traffic signals and grade separated intersections come
under this classification
time sharing approach. At a given time, with the help of appropriate signals,
certain traffic movements are restricted where as certain other movements are
permitted to pass through the intersection. Two or more phases may be provided
depending upon the traffic conditions of the intersection. When the vehicles
traversing the intersection is very large, then the control is done with the
help of signals. The phases provided for the signal may be two or more. If more
than two phases are provided, then it is called multiphase signal.
fixed time signals and vehicle actuated signals. In fixed time signals, the
cycle time, phases and interval of each signal is fixed. Each cycle of the
signal will be exactly like another. But they cannot cater to the needs of the
fluctuating traffic. On the other hand, vehicle actuated signals can respond to
dynamic traffic situations. Vehicle detectors will be placed on the streets
approaching the intersection and the detector will sense the presence of the
vehicleand pass the information to a controller. The controller then sets the
cycle time and adjusts the phase lengths according to the prevailing traffic
Grade separated intersections:
The intersections are of two
types. They are at-grade intersections and grade-separated intersections. In
at-grade intersections, all roadways join or cross at the same vertical level.
Grade separated intersections allows the traffic to cross at different vertical
levels. Sometimes the topography itself may be helpful in constructing such
intersections. Otherwise, the initial construction cost required will be very
high. Therefore, they are usually constructed on high speed facilities like
expressways, freeways etc. These type of intersection increases the road capacity
because vehicles can flow with high speed and accident potential is also
reduced due to vertical separation of traffic.
Grade separated intersections
provided to separate the traffic in the vertical grade. But the traffic need
not be those pertaining to road only. When a railway line crosses a road, then
also grade separators are used. Different types of grade-separators are
flyovers and interchange. Flyovers itself are subdivided into overpass and
underpass. When two roads cross at a point, if the road having major traffic is
elevated to a higher grade for further movement of traffic, then such
structures are called overpass. Otherwise, if the major road is depressed to a
lower level to cross another by means of an under bridge or tunnel, it is
roadways flows at different levels in the grade separated junctions. Common
types of interchange include trumpet interchange, diamond interchange , and
1. Trumpet interchange:
form of three leg interchange. If one of the legs of the interchange meets a
highway at some angle but does not cross it, then the interchange is called
2. Diamond interchange:
form of four-leg interchange found in the urban locations where major and minor
roads crosses. The important feature of this interchange is that it can be
designed even if the major road is relatively narrow.
3. Clover leaf interchange:
It is also a four leg
interchange and is used when two highways of high volume and speed intersect
each other with considerable turning movements. The main advantage of
cloverleaf intersection is that it provides complete separation of traffic. In
addition, high speed at intersections can be achieved. However, the
disadvantage is that large area of land is required. Therefore, cloverleaf
interchanges are provided mainly in rural areas.
definite paths by islands, marking etc. and this method of control is called
channelization. Channelized intersection provides more safety and efficiency.
It reduces the number of possible conflicts by reducing the area of conflicts
available in the carriageway. If no channelizing is provided the driver will
have less tendency to reduce the speed while entering the intersection from the
carriageway. The presence of traffic islands, markings etc. forces the driver
to reduce the speed and becomesmore cautious while maneuvering the
intersection. A channelizing island also serves as a refuge for pedestrians and
makes pedestrian crossing safer.